Agents Window: Do not auto-reopen the auxiliary bar when the window becomes wider

The responsive single-pane layout may automatically hide the auxiliary bar when the Agents Window becomes too narrow. When the window later becomes wider, the auxiliary bar should **not** automatically reopen. Reopening should require explicit user intent.

## Steps to Reproduce

1. Open a workspace session in the Agents Window using the single-pane layout.
2. Show the auxiliary bar/Details area.
3. Resize the window narrower until responsive layout hides the auxiliary bar.
4. Resize the window wider again without interacting with the side-pane sash or layout actions.

## Expected

- The auxiliary bar remains hidden when the window becomes wider.
- The responsive layout does not infer that more width means the user wants the auxiliary bar reopened.
- The user can reopen it explicitly by dragging the side-pane sash or invoking the appropriate show/toggle action.

## Actual

The auxiliary bar can automatically reopen as the window becomes wider, overriding the current hidden layout state.

## Rationale

Responsive behavior should be asymmetric: it may remove UI when space becomes constrained, but it should not restore UI without explicit user intent. This avoids layout jumps while resizing the window.

Related: #330409 tracked a different intermittent narrow-pane symptom and was closed after more information was not provided.